The purpose of this protocol is to provide a mechanism for the Department of Transfusion Medicine, Clinical Center, to perform leukapheresis procedures on subjects participating in an experimental irradiated sporozoite vaccination program for the prevention of plasmodium (malaria) infection, conducted by the Malaria Program at the Naval Medical Research Center (NMRC). The leukapheresis procedures are necessary to enable collection of sufficient numbers of cells for in vitro laboratory studies to follow the effects of the vaccine. The maximum number of subjects who will undergo leukapheresis is 60, 30 vaccinees and 30 nonvaccinated controls, over the course of 10 years, with each subject undergoing apheresis on 4 occasions. The involvement of NIH in this study is the limited to the performance of leukapheresis procedures. The study design, subject recruitment, vaccine development, subject vaccination, laboratory monitoring, and data analysis are performed by NMRC staff, and all study participants must be active-duty employees of the Department of Defense. To date, a total of 29 study subjects have undergone 66 leukapheresis procedures. The procedures consisted of discontinuous flow procedures performed using a Haemonetics MCS Plus device, and processing 4 "passes" per procedure. Mean (+ SD) volume processed was 2155 + 171 mL, with a mean product yield of 4.34 ? 0.77 x 109 total white cells, 82% of which were mononuclear cells (65% lymphocytes, 17% monocytes). Specific cells yields included 2.84 ? 0.59 x 109 lymphocytes and 0.76 ? 0.29 x 109 monocytes. The products also contained a mean of 14 mL of packed red cells and 1.92 x 1011 platelets. No significant adverse effects of apheresis were noted.